Six-Fold-Symmetrical Hierarchical ZnO Nanostructure Arrays

May 12, 2010 - Nanjing 210093, China, and ‡College of Physics, Hohai University, Nanjing ... and the branches epitaxially grow on the backbone along...
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DOI: 10.1021/cg9015367

Six-Fold-Symmetrical Hierarchical ZnO Nanostructure Arrays: Synthesis, Characterization, and Field Emission Properties

2010, Vol. 10 2455–2459

Zhiqiang Wang,† Jiangfeng Gong,†,‡ Yun Su,† Yuwen Jiang,† and Shaoguang Yang*,† †

National Laboratory of Solid State Microstructures, and Department of Physics, Nanjing University, Nanjing 210093, China, and ‡College of Physics, Hohai University, Nanjing 210098, China Received December 8, 2009; Revised Manuscript Received May 6, 2010

ABSTRACT: Herein we present the successful synthesis of aligned arrays of single-crystalline, 6-fold-symmetrical, hierarchical ZnO nanostructures via a two-step vapor-phase transport method. The X-ray diffraction and transmission electron microscopy results illustrate that the backbones of the as-grown product have excellent orientation along the [001] direction, and the branches epitaxially grow on the backbone along the Æ100æ direction. At room temperature, the 6-fold-symmetrical hierarchical ZnO nanostructure arrays exhibit a very strong green luminescence centered at 515 nm. The field emission measurements show that they have a turn-on field of 5.6 V/μm and field-enhancement factor of 1351, which indicate their potential application in field emission nanodevices. Since the discovery of carbon nanotubes,1 enormous efforts have been devoted to researching one-dimensional (1D) nanostructures due to their exciting physical properties and wide applications in electronic, optoelectronic, and electrochemical nanodevices.2-8 Recently, the self-assembly of 1D nanostructures into hierarchical nanoarchitectures as well as nanoheterostructures has become the focus of intensive attention because they may exhibit some novel properties resulting from their unique multidimensional shape and the combination of both micrometer-scale and nanometer-scale building blocks.9-19 Furthermore, they can be used as building units in nanodevices, which is crucial for the development of future nanodevices and nanotechnology. Up to now, the design and synthesis of hierarchical nanostructures with a controlled crystallographic direction and assembling them into arrays remain one of the main challenges to realize functional nanodevices. As one of the most important multifunctional semiconductors, ZnO with a wide band gap energy of 3.37 eV and a large exciton binding energy of 60 meV at room temperature has a wide range of applications in electronic, optoelectronic, electrochemical, and electromechanical devices, such as ultraviolet (UV) lasers,6,8 light-emitting diodes,20 field emission devices,21 solar cells,22 and piezo-nanogenerators.23 In the past few years, hierarchical ZnO micro- and nanostructures with different morphologies were synthesized via gas-phase and solution-phase approaches.10,24-40 Nevertheless, how to assemble the as-prepared hierarchical ZnO nanostructures into arrays with excellent orientation has remained a challenge. In this study, we report the successful fabrication of aligned arrays of hierarchical ZnO nanostructures via a two-step vapor-phase transport method. The as-synthesized product has essential features of nanoscaled, 6-fold-symmetrical hierarchy and single crystallinity. More important, the backbones of the product have excellent orientation along the [001] direction, and the branches epitaxially grow on the backbone along the Æ100æ direction. At room temperature, the photoluminescence (PL) results show that the 6-fold-symmetrical hierarchical ZnO nanostructure arrays have a very strong green emission centered at 515 nm. The field emission (FE) measurements show that they have a turn-on field of ∼5.6 V/μm and field-enhancement factor of 1351, which illustrate their potential application in field emission nanodevices. *Author to whom correspondence should be addressed. Address: National Laboratory of Microstructures, and Department of Physics, Nanjing University, 22# Hankou Road, Nanjing 210093, China. Tel.: þ86-25-83597483. Fax: þ86-25-83595539. E-mail: [email protected].

The synthesis of the 6-fold-symmetrical hierarchical ZnO nanostructure arrays was carried out in a traditional horizontal furnace via a two-step vapor-phase transport method. In the first step, ZnO nanowire arrays which would act as the backbone of the 6-fold-symmetrical hierarchical ZnO nanostructure arrays were prepared. 0.2 g of ZnO (>99.5%) and graphite (>99.85%) powders were mixed with a molar ratio of 1:1 and placed in an alumina boat in the center of a quartz tube, which was inserted into a horizontal tube furnace. A piece of (100) Si wafer coated with a layer of gold thin film (about 10 nm) was used as substrate and put on top of the alumina boat. The temperature of the furnace was raised to 850 C at a rate of 50 C min-1 and typically kept at that temperature for 60 min under a constant flow of argon (4N, 20 sccm). After the furnace was cooled to room temperature, a layer of gray powder was found on the surface of the Si wafer. In the following step, the 6-fold-symmetrical hierarchical ZnO nanostructure arrays were synthesized by thermal evaporating ZnS powder possibly with the presence of residual oxygen. 0.05 g of ZnS powder (>97%) was placed in the center of another quartz tube, which was inserted into the horizontal tube furnace. The ZnO nanowire arrays, which were prepared in the first step, were coated with ∼10 nm Au (5N) and placed downstream in the quartz tube, 8 cm away from the source material. Pure Ar (4N) was used as the carrier gas at 20 sccm. The reaction was carried out at 800 C for 30 min. Then, the furnace was naturally cooled to room temperature. Light gray product was collected on the substrate. The crystal structures and morphologies of the products were characterized by X-ray diffraction (XRD; PANational x’pert) with Cu KR radiation, scanning electron microscopy (SEM; Philips-XL30), and transmission electron microscopy (TEM; Philips Tecnai F20). PL measurement was carried out on a visibleultraviolet spectrophotometer with a 325 nm He-Cd laser as the excitation source at room temperature. FE measurements were carried out in a high-vacuum chamber at a pressure of 2  10-4 Pa at room temperature.41-44 Figure 1a shows the XRD pattern of the ZnO nanowire arrays prepared in the first step (red pattern). Besides the peaks from the Si substrate, other peaks are all indexed to wurtzite-structured ZnO with lattice constants a=0.3253 and c=0.5209 nm (JCPDS 80-0075). The relatively strong (002) peak implies that the ZnO nanowires have a preferred orientation along the c-axis direction. The SEM image (Figure 1b) illustrates that the ZnO nanowires were grown vertically on the Si substrate with a diameter in the range of 90-200 nm and a length of several micrometers. A typical TEM image of the ZnO nanowires is shown in Figure 1c. Two

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Figure 1. (a) XRD patterns of the ZnO nanowire arrays and the 6-fold-symmetrical hierarchical ZnO nanostructure arrays. (b) SEM image of ZnO nanowire arrays prepared in the first step. (c) TEM and (d) HRTEM images of the ZnO nanowires. The inset of panel d is the corresponding SAED pattern of the ZnO nanowire.

straight ZnO nanowires have a diameter of 110 and 140 nm respectively and show a very smooth surface. The selected area electron diffraction (SAED) pattern (inset of Figure 1d) from a single nanowire reveals its single crystal structure, and the corresponding high-resolution transmission electron microscopy (HRTEM) image (Figure 1d) shows clear lattice fringes of the (002) plane of wurtzite ZnO structure. The growth direction of the ZnO nanowire is determined to be [001], which is consistent with the XRD result shown in Figure 1a. The 6-fold-symmetrical hierarchical ZnO nanostructure arrays were fabricated with the ZnO nanowire arrays as substrate. The SEM images of the 6-fold-symmetrical hierarchical ZnO nanostructure arrays are shown in Figure 2. After the second stage of growth, the initially smooth ZnO nanowires branched out and formed hierarchical nanostructures. The nanowire branches have a length of hundreds of nanometers and a diameter of about 40 nm. In the high magnification SEM image (Figure 2b), it can be seen that the hierarchical nanostructures exhibit a significant feature of 6-fold-symmetry. The secondary nanowires 6-foldsymmetrically grew on the ZnO nanowire backbones at the second stage of growth. The XRD pattern of the as-synthesized 6-foldsymmetrical hierarchical nanostructure arrays is shown in Figure 1a (blue pattern), illustrating that the products are hexagonal ZnO. Notice that the intensity ratio of (002) and (100) peaks in the XRD pattern of 6-fold-symmetrical hierarchical ZnO nanostructure arrays is 413, which is much larger than that of ZnO nanowire arrays (18). The reason is proposed to be that a great quantity of horizontal nanowire branches of hierarchical ZnO nanostructures increase the diffraction of the (002) plane. The 6-fold-symmetrical hierarchical ZnO nanostructures were further characterized using TEM. A typical TEM image of the 6-fold-symmetrical hierarchical ZnO nanostructure is shown in Figure 3a. The nanowire branches perpendicularly grew on the side surface of the ZnO nanowire backbone as six rows in a parallel manner. The length and diameter of the nanowire branches are hundreds of nanometers and 25-45 nm, respectively. Besides the nanowire branches, there are some triangle-shaped nanosheet branches standing on the backbone in the same way as the nanowire

Wang et al.

Figure 2. (a) Low and (b) high magnification SEM images of the 6-fold-symmetrical hierarchical ZnO nanostructure arrays. Scale bars in (a) and (b) are 5 and 1 μm, respectively.

Figure 3. (a) TEM image of the 6-fold-symmetrical hierarchical ZnO nanostructures. (b-d) HRTEM images from the nanowire branch (region b), the triangle-shaped nanosheet branch (region c) and the backbone (region d), respectively. The inset of panel d is the corresponding SAED pattern of the 6-fold-symmetrical hierarchical ZnO nanostructures (from region e).

branches. The dark contrast particles on top of the nanowire and nanosheet branches are the gold nanoparticles which were used as catalyst for the growth of the ZnO nanowire branches. It should be noted that some branches were broken off from the trunk

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Figure 4. Schematic diagram showing the two-stage VLS growth of the 6-fold-symmetrical hierarchical ZnO nanostructure arrays. The inset is a cross-sectional model illustrating the isotropic epitaxial growth of the branches around the nanowire backbone.

resulting from the sonication during preparation of the TEM sample. Now let us come closer to the branches and the backbone of the 6-fold-symmetrical hierarchical ZnO nanostructures. The HRTEM images of nanowire branch (Region b in Figure 3a) and triangleshaped nanosheet branch (Region c in Figure 3a) are shown in Figure 3, panels b and c, respectively. The lattice spacing of 0.28 and 0.52 nm corresponds to the (100) and (001) planes of hexagonal ZnO, indicating that both nanowire and nanosheet branches grew along the [100] direction. Figure 3d displays a HRTEM image of the interface section between the backbone and branch (the whole HRTEM image is shown in Figure S1a in Supporting Information). The region with dark contrast corresponds to the nanowire backbone, and that with medium and light contrast corresponds to secondary branches. As shown in Figure S1a, the interface of the branch and backbone is not atomically smooth, because the surface of the ZnO nanowire backbone is not atomically flat (see Figures 1d and S2). The change in contrast in the middle of Figure 3d results from the partial overlap of two adjacent branches of the six branches with 6-fold-symmetry at the same level. The corresponding fast Fourier transform (FFT) pattern (see Figure S1b) clearly shows a set of diffraction spots, indicating that the [001] and [100] directions of the branch are parallel to that of the backbone respectively. The SAED pattern recorded from a much larger area (inset of Figure 3d) is consistent with the FFT result, and further confirms above statements. Therefore, the lateral nanowire branches and the nanowire backbone of 6-fold-symmetrical hierarchical ZnO nanostructures have an epitaxial relationship with identical orientations along the [001] and [100] directions. Normally, the synthesis of hierarchical ZnO nanostructures is achieved by facilitating the growth of lateral branches on the backbone. Some researchers utilize a self-catalyzed growth mechanism to control the growth of secondary branches on ZnO or Zn microprisms.10,26-35,37-40 The other researchers make use of the vapor-liquid-solid (VLS) growth mechanism with Sn as a catalyst to promote the growth of lateral branches on ZnO nanowires and nanorods.24,25 On the basis of SEM and TEM results, we regard that the growth of the as-prepared 6-fold-symmetrical hierarchical ZnO nanostructure arrays is dominated by a twostep VLS mechanism with Au as the catalyst. A schematic illustration of the growth process of the 6-fold-symmetrical hierarchical ZnO nanostructure arrays is shown in Figure 4. At the first VLS growth stage, the Au film on the Si wafer splits into Au liquid droplets at high temperature. The ZnO vapor dissolves in Au liquid droplets continuously. When ZnO becomes supersaturated in Au liquid droplets, ZnO nucleates and grows into nanowire at

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Figure 5. PL spectra of ZnO nanowire arrays and 6-fold-symmetrical hierarchical ZnO nanostructure arrays at room temperature. The inset is a magnification of the band-edge emission at 382 nm.

the liquid/solid interface. At the second VLS growth stage, Au thin film (e10 nm) is coated on the prepared ZnO nanowires in order to facilitate the nucleation and growth of lateral branches. With the increase of substrate temperature, Au thin film on the surface of ZnO nanowires splits into nanosized liquid droplets. At high temperature, ZnS is evaporated and oxidized into ZnO by the residual oxygen in the system. Then, ZnO vapor gradually dissolves in nanosized Au liquid droplets on the surface of ZnO nanowires. The ZnO nanowires are likely to have a hexagonal cross section bounded by ( (1010), ( (0110), and ( (1100), which are six crystallographic equivalent planes.24 Au liquid droplets coated on ZnO nanowire induce the growth of secondary ZnO nanowire branches along the six growth directions: ( [1010], ( [0110], and ( [1100], resulting in the 6-fold-symmetrical distribution of the nanowire branches (see inset of Figure 4). When the growth process is finished, Au liquid droplets solidify into Au nanoparticles standing on top of ZnO nanowire branches, which accords with TEM and HRTEM results (see Figure 3a-c). Here, it should be noted that coating Au thin film on ZnO nanowires is indispensable to grow such 6-fold-symmetrical ZnO nanostructure arrays in our experiments. ZnO nanowires could not branch out to form hierarchical nanostructures without the operation of Au coating in the second growth step (see Figure S3). On the other hand, temperature is an important factor to control the growth of secondary branches. When the temperature is lower than 800 C, the secondary nanowire branches only grow along a part of above six growth directions and do not perform obvious symmetry (see Figure S4). When the temperature is higher than 800 C, large quantities of nanorod branches grow on ZnO backbones (see Figure S5). Most of them are disordered and do not have any symmetry. To investigate the potential applications of the as-grown 6-fold-symmetrical hierarchical ZnO nanostructure arrays in optical and FE nanodevices, the PL and FE measurements were carried out. Room temperature PL spectra of ZnO nanowire arrays and 6-fold-symmetrical hierarchical ZnO nanostructure arrays are shown in Figure 5. The blue plot representing the PL spectrum of ZnO nanowire arrays shows a weak ultraviolet (UV) emission at 382 nm (3.25 eV) and a very strong and broad green emission centered at 515 nm (2.41 eV). The UV emission is attributed to excitonic recombination corresponding to the band-edge emission of ZnO. The green emission is ascribed to the recombination of electrons with holes trapped in singly ionized oxygen vacancies and is commonly observed in ZnO nanostructures synthesized under oxygen-deficient conditions. The large ratio of green PL intensity to band-edge PL intensity (about 78) indicates that the as-prepared ZnO nanowire arrays are rich in atomic defects. The PL spectrum of 6-fold-symmetrical hierarchical ZnO

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Wang et al. The relationship between the current density J and the applied electric field E can be expressed by the Fowler-Nordheim (F-N) equation J ¼ ðAβ2 E 2 =jÞ expð- Bj3=2 =βEÞ

ð1Þ

lnðJ=E 2 Þ ¼ lnðAβ2 =jÞ - Bj3=2 =βE

ð2Þ

or

Figure 6. Field emission from ZnO nanowire arrays and 6-foldsymmetrical hierarchical ZnO nanostructure arrays. (a) J-E curves of ZnO nanowire arrays and 6-fold-symmetrical hierarchical ZnO nanostructure arrays. (b) Fowler-Nordheim plots corresponding to (a).

nanostructure arrays (red plot) has the same features as that of ZnO nanowire arrays, showing a weak UV emission and a very strong and broad green emission centered at 382 and 515 nm, respectively. However, the intensity of the green emission in the red plot is over 4 times larger than that in the blue plot, which may result from the formation of secondary nanowire branches in large quantities. The ratio of green PL intensity to UV PL intensity of 6-fold-symmetrical hierarchical ZnO nanostructure arrays is as high as 640, which is over 8 times larger than that of ZnO nanowire arrays. Such a tremendous ratio of green PL intensity to UV PL intensity of the 6-fold-symmetrical hierarchical ZnO nanostructure arrays is because the growth of secondary nanowire branches in large quantities under oxygen-deficient conditions resulted in an extreme increase of oxygen vacancies on the surface of the 6-fold-symmetrical hierarchical ZnO nanostructures. Figure 6a shows the field emission current density as a function of the applied electric field (J-E) for ZnO nanowire arrays and 6-fold-symmetrical hierarchical ZnO nanostructure arrays. The turn-on field (Eto) and the threshold field (Ethr) are defined as the electric fields that produce emission current density of 10 μA/cm2 and 1 mA/cm2, respectively. Eto and Ethr of ZnO nanowire arrays are about 6.1 and 9.2 V/μm and that of 6-fold-symmetrical hierarchical ZnO nanostructure arrays are about 5.6 and 9.3 V/μm. Six-fold-symmetrical hierarchical ZnO nanostructure arrays perform a lower turn-on field than ZnO nanowire arrays, because the average diameter of nanowire branches in hierarchical ZnO nanostructures is much smaller than that of ZnO nanowire arrays.

where A=1.5610-10 A eV V-2, B=6.83103 V eV-3/2 μm-1, β is the field-enhancement factor, and j is the work function of the emitting materials, which is 5.3 eV for ZnO. The F-N plots of ln(J/E2) versus 1/E are shown in Figure 6b. The linear characteristic of the F-N curves within the measurement range confirm that the electron emission current of ZnO nanowire arrays and 6-fold-symmetrical hierarchical ZnO nanostructure arrays truly results from field emission. Notice that the FN plot of hierarchical ZnO nanostructure arrays (plotted by red triangles) is not perfectly linear and shows a deviation from linearity at low electric field region. Such phenomenon in the FN plot is also reported in other types of field emitters.45-47 In general, for ZnO nanostructures, the slope of FN plot shows a large value at a low applied field and a small value at a high field. However, the mechanism of such phenomenon is not clear yet. There are many hypotheses to attribute the origin to space charge effect, localized state, and adsorbate-enhanced tunnelling state. The field-enhancement factor β, which is related to the emitter geometry, crystal structure, and spatial distribution of the emitting centers, is calculated to be about 1397 and 1351 for ZnO nanowire arrays before and after growth of secondary branches, respectively. The calculated values are much higher than that (β= 50-100) estimated from the sample geometry obtained from SEM and TEM images. Similar results are also found by other researchers.48,49 The reason for the increase of field-enhancement factor may be ascribed to the microscopic structure of the emitting surface, sharp corners at the nanowire tips, or field effect due to adjacent nanowires. Normally, the decrease of diameter of nanowires leads to the increase of field-enhancement factor. In our experiments, the diameter of nanowire branches is 25-45 nm, which is much smaller than that of nanowire backbones (100-200 nm). Therefore, it can be expected that the field-enhancement factor of 6-fold-symmetrical ZnO nanostructure arrays is larger than that of ZnO nanowire arrays. Actually, the field-enhancement factor of hierarchical ZnO nanostructure arrays (1351) is little smaller than that of ZnO nanowire arrays (1397). The abnormal behavior may originate from a shielding effect which occurs between closely packed nanowire branches and lowers the field-enhancement factor of hierarchical ZnO nanostructures. In summary, novel hierarchical ZnO nanostructure arrays were successfully fabricated via a two-step vapor-phase transport approach. The product exhibits important advantages such as 6-fold-symmetrical branches, single crystallinity, and excellent orientation, which suggests that it can be used as promising nanometerscaled building blocks. At room temperature, the 6-fold-symmetrical hierarchical ZnO nanostructure arrays show strong green luminescence and good field emission properties, indicating their potential applications in optical and field emission nanodevices. Acknowledgment. The authors thank Prof. Song Fengqi for his kind help in the measurements and analysis of the FE properties of the samples. This work was supported by Program for New Century Excellent Talents in University (07-0430), National Basic Research Program of China (2007CB936302), China Postdoctoral Sustentation Fund (Grant No. 200904501062), and Jiangsu Province Postdoctoral Sustentation Fund (Grant No. 0901001B). Supporting Information Available: The whole HRTEM image of interface section between backbone and branch of the hierarchical

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ZnO nanostructures and corresponding FFT pattern. HRTEM image of ZnO nanowire. SEM images of ZnO nanostructure arrays synthesized under different growth conditions. This information is available free of charge via the Internet at http://pubs.acs.org.

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